1. Field of the Invention
The present invention relates to current amplifiers, and more particularly to amplifiers for amplifying low currents with a large band-width.
2. Description of the Related Art
FIG. 1 represents such a current amplifier in a conventional application. The current to be amplified i is provided to an input A of the amplifier through a photodiode D connected to a supply voltage Vcc. When the photodiode D is, for example, used in a compact disk (CD) read head, the variations in the light intensity, and therefore the variations in the current of photodiode D, are very low. The maximum value of current i is approximately 3 μA. Furthermore, current i may vary at a frequency of approximately 1.5 MHz.
A parasitic capacitance Cp1 is present between the input A of amplifier 10 and ground GND, and a parasitic capacitance Cp2 is present between the input A and the supply voltage Vcc. These two parasitic capacitances Cp1 and Cp2 may significantly reduce the band-width of amplifier 10 and inject noise from the supply voltages in the input. To avoid these drawbacks, the input impedance of amplifier 10 must be reduced or, in other terms, the voltage of the input must be as steady as possible.
FIG. 2 represents an exemplary conventional current amplifier providing satisfactory performance for a photodiode of a CD read head. In practice, the current amplifier 10 is a current mirror with bipolar transistors. An NPN transistor Q1 forms the input transistor of the current mirror. The emitter of transistor Q1 is connected to ground GND and its base and collector receive from the photodiode D the current i to amplify. Furthermore, a constant current source 12, connected in parallel with diode D, provides a bias current I. An NPN transistor Q2, forming an output transistor of the current mirror, is connected in parallel by its base and emitter to transistor Q1. The output current of the amplifier is drawn from the collector of transistor Q2. The emitter surfaces of transistors Q1 and Q2 are indicated in brackets. Transistor Q1 has a unity surface and transistor Q2 has a surface n. Thus, the current i in photodiode D is amplified by a factor n at the collector of transistor Q2.
Such a current amplifier, formed of bipolar transistors, has an input impedance of approximately 1 kΩ, which is sufficient to amplify currents of approximately 3 μA with a band-width of approximately 1.5 MHz in common cases where the parasitic capacitances are relatively low.
The digital circuits for processing the signal provided by the photodiode D, in particular for CD's, are generally realized in CMOS technology, which is inexpensive and well adapted to digital processing.
The performance of a current mirror realized in CMOS technology is insufficient to amplify low currents with a large band-width. So, in CD players, the current amplifiers of the photodiodes of the read heads are realized in bipolar technology and cannot be integrated in the same circuit as the signal processing circuit.
FIG. 3 illustrates another approach to implementing a current amplifier for CD applications in CMOS technology. This approach is disclosed in U.S. Pat. No. 5,867,066 issued on Feb. 2, 1999, to Dell'Ova, et al, and entitled “Current Amplifier” (“the '066 patent”). A circuit which has improved temperature characteristics beyond those of the '066 patent is preferred.